1. Field of the Invention
The present invention relates to a method used in a wireless communications system and related communication device, and more particularly, to a method of handling random access channel (RACH) procedures in a wireless communications system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) for communicating with a plurality of user equipments (UEs) and communicates with a core network including a mobility management entity (MME), a serving gateway, etc. for NAS (Non Access Stratum) control.
An LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), uplink (UL) multiple-input multiple-output (MIMO), etc.
For bandwidth extension, the carrier aggregation (CA) technology is introduced to the LTE-A system by which two or more component carriers are aggregated to achieve a wider-band transmission. Accordingly, the LTE-A system can support a wider bandwidth up to 100 MHz by aggregating a maximum number of 5 component carriers, where bandwidth of each component carrier is 20 MHz and is backward compatible with 3GPP Rel-8.
When the UE is configured with the CA, the UE is allowed to receive and transmit data on one or multiple component carriers to increase the data throughput. In the LTE-A system, the component carriers configured to the UE have to consist of one downlink primary component carrier (DL PCC) and one uplink primary component carrier (UL PCC). Component carriers other than the primary component carriers are named UL or DL secondary component carriers (SCCs). A cell operating on the primary component carriers is known as a primary cell (PCell), which handles radio resource control (RRC) connection establishment/re-establishment/handover. A cell operating on the secondary component carrier is known as a secondary cell (SCell), which may be configured after an RRC connection is established in order to provide additional radio resources. The PCell (i.e. the UL and DL PCCs) is always activated, whereas the SCell may be activated or deactivated according to specific conditions (e.g. an amount of data for transmission).
A random access channel (RACH) procedure is performed by a UE to acquire uplink (UL) synchronization. The UE performs the RACH procedure by transmitting random access preamble(s) to the target eNB, receiving a corresponding random access response from the target eNB, and validating a preamble number included in the random access response. Accordingly, the UE transmits a resource request or a handover complete to the target eNB using resources indicated in the random access response. After a grant corresponding to the resource request or the handover complete is received from the target eNB, the UE is able to transmit and receive data. Therefore, it is important for the UE to complete the RACH procedure.
In the LTE-A system, the UE is allowed to perform a network initiated RACH procedure on the SCell. For example, the eNB may configure a new serving cell (i.e., a SCell) to the UE to improve data throughput and activate the SCell by sending a media access control (MAC) control element (CE). In order for the UE to obtain uplink synchronization of the SCell, the eNB may then send a physical downlink control channel (PDCCH) order to the UE to trigger a RACH procedure on the SCell.
The eNB may send an information request message to the UE for obtaining a RACH report, so as to help the eNB to optimize RACH configurations of the succeeding RACH procedure. The RACH report may include a parameter to represent the amount of random access preambles which have been sent by the UE for the last successfully completed RACH procedure, thereby the eNB can optimize the RACH configurations accordingly. Since the UE is allowed to perform a network initiated RACH procedure on the SCell, the last serving cell on which the UE completed the RACH procedure successfully could be a SCell. However, the SCell may have been de-configured or deactivated when the UE receives the information request message from the eNB, and therefore the amount of random access preambles included in the RACH report may be obsolete or futile for the optimization. In the worst case, reporting such information may mislead the eNB to perform a wrong optimization for the RACH procedure on the PCell.
Thus, how to improve the performance of the RACH procedures for the CA technology is a topic to be addressed and discussed in the industry.